Spectroscopic imaging combines digital imaging and molecular spectroscopy techniques, which can include Raman scattering, fluorescence, photoluminescence, ultraviolet, visible and infrared absorption spectroscopies. When applied to the chemical analysis of materials, spectroscopic imaging is commonly referred to as chemical imaging. Instruments for performing spectroscopic (i.e. chemical) imaging typically comprise an illumination source, image gathering optics, focal plane array imaging detectors and imaging spectrometers.
In general, the sample size determines the choice of image gathering optic. For example, a microscope is typically employed for the analysis of sub micron to millimeter spatial dimension samples. For larger objects, in the range of millimeter to meter dimensions, macro lens optics are appropriate. For samples located within relatively inaccessible environments, flexible fiberscope or rigid borescopes can be employed. For very large scale objects, such as planetary objects, telescopes are appropriate image gathering optics.
For detection of images formed by the various optical systems, two-dimensional, imaging focal plane array (FPA) detectors are typically employed. The choice of FPA detector is governed by the spectroscopic technique employed to characterize the sample of interest. For example, silicon (Si) charge-coupled device (CCD) detectors or CMOS detectors are typically employed with visible wavelength fluorescence and Raman spectroscopic imaging systems, while indium gallium arsenide (InGaAs) FPA detectors are typically employed with near-infrared spectroscopic imaging systems.
Spectroscopic imaging of a sample can be implemented by one of two methods. First, a point-source illumination can be provided on the sample to measure the spectra at each point of the illuminated area. Second, wide-field illumination may be used to collect spectra over the entire area encompassing the sample simultaneously using an electronically tunable optical imaging filter. Here, the organic material in such optical filters are actively aligned by applied voltages to produce the desired bandpass and transmission function. The spectra obtained for each pixel of such an image thereby forms a complex data set referred to as a hyperspectral image which contains the intensity values at numerous wavelengths or the wavelength dependence of each pixel element in this image.
Spectroscopic devices operate over a range of wavelengths due to the operation ranges of the detectors or tunable filters possible. This enables analysis in the ultraviolet (UV), visible (VIS), near infrared (NIR), short-wave infrared (SWIR), extended range SWIR, mid infrared (MIR) wavelengths, and to some overlapping ranges. These correspond to wavelengths of about 180-380 nm (UV), about 380-700 nm (VIS), about 700-2500 nm (NIR), about 900-1700 nm (SWIR), about 1200-2500 nm (extended range SWIR) and about 2500-25000 nm (MIR).
There currently exists a need for accurate detection of unknown materials which may be threat materials such as biological, chemical, hazardous, and drug materials (including pharmaceutical and illegal drugs). Such a system and method may hold potential for detecting threat and other materials associated with individuals and their belongings in a wide variety of settings including, but not limited to: security checkpoints, points of inspection, transportation terminals, stadiums, traffic stops, security monitoring and other similar situations. There also exists a need for a system and method for the detection of such materials located in or on a person or an article associated with that person, including clothing items.